Pierre Boulez, the French composer and conductor who helped blaze a radical new path for classical music in the 20th century, becoming one of its dominant figures in the decades after World War II, died on Tuesday at his home in Baden-Baden, Germany. He was 90.

His family confirmed his death in a statement to the Philharmonie de Paris. Prime Minister Manuel Valls, also in a statement, said, “Audacity, innovation, creativity — that is what Pierre Boulez was for French music, which he helped shine everywhere in the world.”

Mr. Boulez belonged to an extraordinary generation of European composers who emerged in the postwar years while still in their 20s. They started a revolution in music, and Mr. Boulez was in the front ranks.

As a young composer — and throughout his life as an insistently private man — he matched restless intelligence with great force of mind: He knew what had to be done, by his reading of history, and he did it, in defiance of all the norms of French musical culture at the time. His “Marteau Sans Maître” (“Hammer Without a Master”) was one of this pioneering group’s first major achievements, and it remains a landmark of modern music.

But his influence was equally large on the podium. In time he began giving ever more attention to conducting, where his keen ear and rhythmic incisiveness could produce a startling clarity. (There are countless stories of him detecting faulty intonation, say, from the third oboe in a complex piece.)

He reached his peak as a conductor in the 1960s, when he began to appear with some of the world’s great orchestras, like the Concertgebouw in Amsterdam, the Berlin Philharmonic and the Cleveland Orchestra. By the early ’70s, he had succeeded Leonard Bernstein as music director of the New York Philharmonic, an appointment that startled the music world and led to a fitful tenure.

His conducting style was unique. He never used the baton, preferring to manipulate the orchestra by means of his two hands simultaneously, the left indicating phrasing or, in much contemporary music, counterrhythm.

His characteristic sound — unemotional on the surface but with undercurrents of intemperateness, at once brilliant in color and rhythmically disciplined — depended on his famously acute ear and suited his core repertoire: Stravinsky (several of whose works he introduced to Europe), Debussy, Webern, Bartok and Messiaen. It was refreshing as well in his many excursions into earlier music.

To be a conductor, though, meant working with the existing machinery, and that was not something a revolutionary like Mr. Boulez was willing to do. So he tried to remake the machinery. After becoming music director of both the New York Philharmonic and the BBC Symphony Orchestra in London in 1971, he explored unconventional repertoire, unconventional concert formats and unconventional locations.

But he also accepted that he had to rethink some of his own preconceptions, and as his musical outlook broadened, his output as a composer dwindled.

It was his reputation as an avant-garde composer and as a champion of new music that prompted his unexpected appointment in New York. After the initial shock at his arrival, there was hope that he might bring the orchestra into the 20th century and appeal to younger audiences. But his programming often met with hostility in New York, and he left quietly six years later.

His destination was Paris. Dismissive of the French musical establishment, he had spent most of the previous two decades abroad, but President Georges Pompidou, keen to reclaim a native son, had agreed to found a contemporary-music center for him in the capital: the Institute for the Research and Coordination of Acoustics and Music, known as Ircam. It had its own 31-piece orchestra, the Ensemble Intercontemporain.

Mr. Boulez gained further government support in the ’80s, when he achieved his grandest project, the City of Music complex in Paris, housing the Paris Conservatoire, a concert hall and an instrument museum.

Pierre Boulez (the Z is not silent) was born on March 26, 1925, in Montbrison, a town near Lyon, the son of an industrialist, Léon Boulez, and the former Marcelle Calabre. He studied the piano and began to compose in his teens.

A defining moment came when he heard a broadcast of Stravinsky’s “Song of the Nightingale” conducted by Ernest Ansermet; it was a work to which he often returned throughout his conducting career. Rejecting the wishes of his father, who wanted him to study engineering, he went to Paris in 1942 and enrolled at the Conservatoire.

Picking Up the Torch

In 1944-45, he took a harmony class taught by Olivier Messiaen, whose impact on him was decisive. Messiaen’s teaching went far beyond traditional harmony to embrace new music that was outlawed both by the stagnant Conservatoire and by the German occupying forces: the music of Schoenberg, Stravinsky, Bartok and Webern.

Messiaen also introduced his students to medieval music and the music of Asia and Africa. Mr. Boulez felt his course was set; but he also knew he needed to go further into the 12-tone method that Schoenberg had introduced a generation before.

“I had to learn about that music, to find out how it was made,” he once told Opera News. “It was a revelation — a music for our time, a language with unlimited possibilities. No other language was possible. It was the most radical revolution since Monteverdi. Suddenly, all our familiar notions were abolished. Music moved out of the world of Newton and into the world of Einstein.”

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Pierre Boulez leading the Met Orchestra at Carnegie Hall in New York in 2010.CreditHiroyuki Ito for The New York Times

To start on this route, he took lessons in 1945-46 with René Leibowitz, a Schoenbergian who had settled in Paris. Soon, in works like his mighty Second Piano Sonata (1947-48), he was integrating what had been separate paths of development in the music of the previous 40 years: Schoenberg’s serialism, Stravinsky’s rhythmic innovations and Messiaen’s enlarged notion of mode.

As Mr. Boulez saw it, all these composers had failed to pursue their most radical impulses, and it fell to a new generation — specifically, to him — to pick up the torch.

Though he was outspoken about his historical role, he was much warier of talking about what his music expressed. There was the odd reference in his early writings to the poet and playwright Antonin Artaud; there was also an admitted kinship with the poetry of René Char, which he set to music in “Le Marteau Sans Maître” and other works. But he was also capable of ferocious abstraction, as in the first section of his “Structures” (1951) for two pianos, a test case in applying serial principles to rhythm, volume and color.

About his private life, he remained tightly guarded. Jeanne, his older sister, was important to him; few others were able to break through his reserve.

At the beginning of his career, he was hired as music director of a theater company in Paris run by Jean-Louis Barrault and Madeleine Renaud. His 10-year appointment with them was crowned in 1955 by a production of “The Oresteia” of Aeschylus, for which he wrote an ambitious score; they also helped him set up the Domaine Musical concerts in 1953.

The Domaine Musical, intended as a platform for new music, 20th-century classics and early music that was little performed, proved Mr. Boulez’s abilities as an administrator and, later, as a conductor. It also provided a model of the contemporary ensemble that was widely imitated and has remained central to the propagation of new music.

Mr. Boulez made his debut as a concert conductor on March 21, 1956, at a Domaine Musical concert. (The organization was still known then as the Concerts du Petit Marigny, after the theater in Paris in which the concerts took place.) The program included “Le Marteau Sans Maître,” which had received its first performance the previous summer in Baden-Baden.

At once delectable and stringent, this work united traditions of Austrian-German discipline and French finesse with the sounds of Africa, East Asia and South America, made available by its variegated ensemble; besides contralto voice, it included alto flute, viola, guitar and percussion.

“Le Marteau Sans Maître” was widely admired, not least by Stravinsky, who heard it when Mr. Boulez made his North American debut in Los Angeles in March 1957.

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Mr. Boulez in 1971.CreditLarry C. Morris/The New York Times

Mr. Boulez gave his first concert with a symphony orchestra in June 1956, when he conducted the Orquesta Sinfonica Venezuela on one of his last tours with the Renaud-Barrault company. During the 1957-58 season, he appeared with the West German Radio Symphony in Cologne in his own “Visage Nuptial” and Stockhausen’s “Gruppen.”

He then forged a lasting connection with the Southwest German Radio Symphony Orchestra of Baden-Baden, where he made his home. In 1960, he conducted the orchestra in the first performance of his “Pli Selon Pli,” an hourlong setting of poems by Stéphane Mallarmé for soprano, with an orchestra rich in percussion.

That lustrous score allowed the conductor certain flexibilities in assembling its fragments. A musical work should be a labyrinth, with no fixed route, Mr. Boulez often said. It might also never have a fixed ending. From then on, he began starting more works than he ever brought to completion, while at the same time submitting older pieces to rounds of revision.

As a conductor, he showed much less hesitation. Where his first concerts had been devoted entirely to 20th-century works, he began, in the early 1960s, to explore earlier repertoires — Haydn, Bach, Schubert, Mozart, Beethoven — with the Concertgebouw and the Southwest German Radio Symphony Orchestra. He made his debut with an American orchestra, the Cleveland Orchestra, in March 1965. The program, a typical one for him, comprised Rameau, his own music (“Figures-Doubles-Prismes”), Debussy and Stravinsky (“The Song of the Nightingale”).

The next year, he conducted his first operas, “Wozzeck” in Frankfurt and Paris, and “Parsifal” at Bayreuth in Germany, and he started recording for Columbia Masterworks. His first releases for the label included “Wozzeck” and albums of Debussy and Messiaen.

Resistance in New York

His appointment to the New York Philharmonic in 1971 presented great challenges. As music director, he had to enlarge his repertoire rapidly. Until then, he had conducted very little Romantic music other than Berlioz’s; now Schubert, Brahms, Dvorak and Borodin joined his programs, not always convincingly. Though he refused to compromise on Tchaikovsky, he was becoming much more like a regular conductor.

Part of his individuality was lost in the colossal task of maintaining important positions on both sides of the Atlantic, his post with the BBC Symphony demanding much of his time as well. Added to the load was his commitment to prepare the Bayreuth centenary “Ring” in 1976.

Both his programming and his handling of an older repertoire met with some resistance from audiences, critics and, it was said, even some of his musicians. Harold C. Schonberg of The New York Times called Mr. Boulez “a brainy orchestral technician” whose “scientific approach” lacked heart. Reviewing a 1972 concert that included Edgard Varèse’s 1927 composition “Arcana,” Donal Henahan of The Times reported that “perhaps a quarter of the downstairs audience” at Philharmonic Hall “fled as if from the Black Death” before the piece was performed.

Mr. Boulez wanted to make the orchestra a more flexible institution, and a more modern one. Performances might begin with short programs of chamber music, played by members of the orchestra. More of the repertoire would be explored. During his first season as the music director, there was an emphasis on Liszt. Then concerts consisting entirely of new and recent works were given at downtown sites. There were also “informal evenings” of talk, rehearsal and performance featuring 20th-century composers. And there were summer seasons of “rug concerts,” with a different program every night for a week, played to audiences seated on the floor of Philharmonic Hall.

The rug concerts lasted only two years, and none of his other innovations survived his departure. He had given up his post with the BBC Symphony in 1975, leaving as a parting gift his somber “Rituel.” His last concerts with the Philharmonic were in May 1977; on the program was Berlioz’s “Damnation of Faust.” He went back regularly to conduct in London, but he did not return to the Philharmonic podium until 1986.

His priority after the Philharmonic was Ircam, the Paris research institute, and he cut back on his conducting commitments; among the few he kept was the first full performance of Berg’s “Lulu” in 1979, at the Paris Opera.

Believing that music’s development since 1945 had been frustrated by a lack of research into electronic possibilities, Mr. Boulez set to work at Ircam on “Répons,” for a small orchestra with six percussion soloists whose sounds are digitally transformed and regenerated. It was first performed in October 1981.

The paradox was that the man who had such an extraordinary orchestral imagination — and such extraordinary powers to realize the fruits of that imagination in performance — should have been so convinced of his need for electronic resources. “Répons” is in most respects inferior to “Éclat/Multiples,” a work for a similar percussion-based orchestra that he had begun and abandoned a decade before. Nor does it begin to rival the orchestral virtuosity displayed in the arrangements of his early piano cycle “Notations.”

He continued to add to “Répons” during the early 1980s, though much of his creative energy was going into new versions of old scores. In the early 1990s, he emerged from his tumult of rewriting to produce at Ircam the greatest of his late works, a new version of “explosante-fixe” initially conceived as a memorial to Stravinsky — for electronic flute and small orchestra.

He also began to appear more widely again as a conductor, with orchestras in the United States (Los Angeles, Cleveland, Chicago) and Europe. (The concerts were often associated with recording sessions for Deutsche Grammophon.) He returned to what had always been his main repertoire, while also developing enthusiasm for Mahler and making occasional visits to territory he had not touched before: Richard Strauss, Bruckner, Scriabin, Janacek.

At his death, he was conductor emeritus of the Chicago Symphony Orchestra. Among the honors Mr. Boulez received in his later years were the Kyoto Prize in 2009 and the BBVA Foundation Frontiers of Knowledge Award in 2013. He was named a professor at the Collège de France. He won dozens of Grammy Awards.

“He never ceased to think about subjects in relation to one another; he made painting, poetry, architecture, cinema and music communicate with each other, always in the service of a more humane society,” the office of President François Hollande said in a statement.

In 1995, his 70th-birthday year, Mr. Boulez conducted his own and other 20th-century music in London, Paris, Vienna, New York, Tokyo, Amsterdam, Brussels and Chicago. In 2005, he spent his 80th birthday in Berlin at a retrospective of his music. A few pieces were completed in this period, notably “Dérive 2,” a 45-minute score for 11 instruments that took almost two decades to reach its end point, in 2006.

Many more projects remained unfinished, while others were never begun, like the opera on which he was to have collaborated first with Jean Genet and later with Heiner Müller.

Even so, the achievements embodied in his published works and recordings are formidable, and his influence was incalculable. The tasks he took on were heroic: to continue the great adventure of musical modernism, and to carry with him the great musical institutions and the widest possible audience.

Aurelien Breeden contributed reporting from Paris.

Aurelien Breeden contributed reporting from Paris.

A version of this article appears in print on , on Page B15 of the New York edition with the headline: Pierre Boulez, Music’s Uncompromising Modernist, Dies at 90. Order Reprints | Today’s Paper | Subscribe